Promotion of Reaction within the Metal-Bound Ligand

The iron enzymes catalase and peroxidasepromote these reactions very effectively, but it is unlikely that both reactants are coordinated to the metal during the reaction. It is generally true that the metal site is restricted in enzymes, allowing coordination of only one reactant. The other reactant is however often held close by the protein structure. 

The Schiff base can undergo a variety of reactions in addition to transamination, shown in Fig. 6.4 for example, racemization of the amino acid via the a-deprotonated intermediate. Many of these reactions are catalyzed by metal ions and each has its equivalent nonmetallic enzyme reaction, each enzyme containing pyridoxal phosphate as a coenzyme. Many ideas of the mechanism of the action of these enzymes are based on the behavior of the model metal complexes. 

2 Promotion of Reaction within the Metal-Bound Ligand 

The shift of electrons away from the ligand, which is usually induced by the positive charge of the metal, will lead to accelerated nucleophilic attack at the ligand. Since nucleophiles donate electrons they thus act to redress the balance. This effect may well reinforce the neighboring group effect. Some examples of its operation are shown in Table 6.2. All the reactions are second-order. 

FefCNljNO - OH- FelCNfjNOj- via FelCNljNOjH - metal polarization causes ligand to be present as NO+ in substrate a 

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Promotion of Reaction within the Metal-Bound Ligand 307 Table 6.2 Some Examples of Metal-Promoted Reactions... 

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